As new user to Calibre I was investigating this subject (but because I wanted it larger, hihi) ... for those looking into it in future:

for me worked just to run E-book viewer from start menu (ie. just run it empty), check "Remember window size" in preferences (along with other settings I wanted as defaults), and then close it. It worked same way as if "setting defaults" function. Hinted to do this by sherman in this thread

As a new user please don't start answering every question in 18 month old threads. Calibre was at least 70 versions removed from where it is now and answering these is counter productive and just clutters up the forum.

Of course if you have a question, tip, statement or experience you'd like to share feel free to start a new thread (see attached).